patch 8.2.4241: some type casts are redundant

Problem:    Some type casts are redundant.
Solution:   Remove the type casts. (closes #9643)
diff --git a/src/sign.c b/src/sign.c
index 1be1d13..e1aae51 100644
--- a/src/sign.c
+++ b/src/sign.c
@@ -1745,38 +1745,38 @@
 {
     char_u	*p;
 
-    dict_add_string(retdict, "name", (char_u *)sp->sn_name);
+    dict_add_string(retdict, "name", sp->sn_name);
     if (sp->sn_icon != NULL)
-	dict_add_string(retdict, "icon", (char_u *)sp->sn_icon);
+	dict_add_string(retdict, "icon", sp->sn_icon);
     if (sp->sn_text != NULL)
-	dict_add_string(retdict, "text", (char_u *)sp->sn_text);
+	dict_add_string(retdict, "text", sp->sn_text);
     if (sp->sn_line_hl > 0)
     {
 	p = get_highlight_name_ext(NULL, sp->sn_line_hl - 1, FALSE);
 	if (p == NULL)
 	    p = (char_u *)"NONE";
-	dict_add_string(retdict, "linehl", (char_u *)p);
+	dict_add_string(retdict, "linehl", p);
     }
     if (sp->sn_text_hl > 0)
     {
 	p = get_highlight_name_ext(NULL, sp->sn_text_hl - 1, FALSE);
 	if (p == NULL)
 	    p = (char_u *)"NONE";
-	dict_add_string(retdict, "texthl", (char_u *)p);
+	dict_add_string(retdict, "texthl", p);
     }
     if (sp->sn_cul_hl > 0)
     {
 	p = get_highlight_name_ext(NULL, sp->sn_cul_hl - 1, FALSE);
 	if (p == NULL)
 	    p = (char_u *)"NONE";
-	dict_add_string(retdict, "culhl", (char_u *)p);
+	dict_add_string(retdict, "culhl", p);
     }
     if (sp->sn_num_hl > 0)
     {
 	p = get_highlight_name_ext(NULL, sp->sn_num_hl - 1, FALSE);
 	if (p == NULL)
 	    p = (char_u *)"NONE";
-	dict_add_string(retdict, "numhl", (char_u *)p);
+	dict_add_string(retdict, "numhl", p);
     }
 }